Explaining rainfall and vegetation gradients along the eastern slopes of the Bolivian Andes using SALLJEX observations and WRF simulations

During the recent South American Low-Level Jet EXperiment (SALLJEX) special aircraft measurements were made along the eastern slopes of the Bolivian Andes and the bordering lowlands, near the sharp change in the orientation of the Andes (the “bend”) from NW-SE to N-S (near the city of Santa Cruz). Observations from a NOAA WP-3D research aircraft showed large horizontal wind gradients on many days and a pronounced mesoscale warm feature in the lower troposphere, associated with cloud-free conditions. Since this cloud-free zone is commonly observed under the frequent northwesterly wind conditions over the region we decided to explore the dynamics of the mean-flow/topography interaction with the aim of explaining the strong climatological rainfall gradient found near Santa Cruz. Annual rainfall on the low-lying ground away from the foothills ranges from more than 2000 mm about 100km northwest of Santa Cruz to less than 800 mm 100km south of Santa Cruz.

We use the WRF model with grid spacing of ~4 km to model the mesoscale structure of the northwesterly flow, selecting a number of cases with initial conditions from the AVN global output. The flow simulations will be compared with the WP-3D observations for general agreement. We will describe the mean vertical motion field from a composite of selected simulations, with the aim of explaining the mean cloud fields observed in visible and infrared satellite imagery composites. This in turn, should help to explain the observed rainfall distribution, and thus the corresponding vegetation patterns.